Every day millions of passengers put their lives in the hands of train conductors, bus drivers and the truck drivers who share the road with them. All of these jobs require long, monotonous hours with few breaks in between, which can sometimes lead to drivers falling asleep at the wheel.

When that happens, the results can be catastrophic. While some accidents are caused by simple fatigue, others are caused by an easily diagnosed and treated medical condition called sleep apnea.

Four people died and 63 were injured when a train on New York City's Metro North line crashed outside of the city after the train took a turn too fast and slipped off the rails. The cause of the accident was determined to be the engineer suffered from sleep apnea, which was undiagnosed.

Sleep apnea causes the windpipe to constrict during sleep, waking you up temporarily and preventing deep sleep. Elderly people and those who are overweight are at higher risk.

National Transportation Safety Board member Mark Rosekind said sleep apnea is a national problem with drivers and conductors.

Some companies are taking steps to combat this problem. Trucking company Prime has its own sleep lab to screen its drivers for sleep apnea. But so far, there are no nationwide rules to require drivers to be tested, which transportation experts say is a major and dangerous issue.

"One of the biggest problems with that is that many of those people don't know they've got it," Rosekind said.

Former school bus driver Diesha Clay, 30, of Charlotte, North Carolina, was one such driver. Clay was caught on camera falling asleep at the wheel of a school bus full of kids. Clay said she doesn't remember falling asleep, but she was immediately terminated 30 days after the video became public. She was never tested for sleep apnea before or after the accident, until "Nightline" brought her to Carolinas Healthcare System in Charlotte, North Carolina for a test.

"If I do have it, I would be a mixed feeling," Clay said before the test. "I will be happy to know cause then I know what to expect and what I need to do. And then also confused with like, if I would've known before, I don't think I would be in this situation."

To be tested for sleep apnea, a registered sleep technologist monitored Clay overnight, and then Dr. Jaspal Singh reviewed the results with her the following morning. Singh told her she stopped breathing roughly seven times an hour, meaning Clay had "mild to moderate" sleep apnea.

She was given a machine called a CPAP (Continuous Positive Airway Pressure) to wear when she sleeps and help regulate her breathing. Since her diagnosis, Clay has contacted a Human Resources lawyer in the hopes that she may be able to challenge her termination, due to the fact that she had undiagnosed sleep apnea. Clay did not initially challenge her termination.

"To think about the people who don't know they have sleep apnea, it's scary because honestly some people do not know," she said. "For example, like me. I did not know. I didn't have a clue. … In my opinion, I feel like there should be wide testing when it comes to transportation."

But the NTSB says their recommendations for national testing of train conductors, school bus drivers, pilots and truckers are not being acted on fast enough.

"When the NTSB investigates we try to make recommendations so that horrible crashes don't happen again, and so we're going to tell agencies, 'you have to address sleep apnea,'" Rosekind said.

The Department of Transportation declined our request for an interview, but in a statement said, "Safety is the top priority at the Department of Transportation and we recognize that sleep apnea can have a direct effect on those responsible for transporting goods and people. DOT is committed to ensuring that all train operators, truck drivers and pilots are fit for the job and receive the treatment they need to combat sleep apnea."

While critics say nationwide rules for sleep apnea testing are needed immediately, others are not convinced sleep apnea is the sole cause of accidents.

Todd Spencer, the executive vice president of the Owner-Operator Independent Drivers Association (OOIDA), said the theory that sleep apnea causes accidents is "junk science."

"We've not see the impact in safety and health [of sleep apnea] that [the NTSB] talks about," Spencer said. "Generally when somebody's saying, 'This is--we're killing people, we're killing people,' they have an economic interest in pursuing this… either they're make money from treatment, they make money from diagnosis, they make money maybe through lawsuits."

But the DOT said one of its agencies, the Federal Motor Carrier Safety Administration, has begun research that could one day lead to sleep apnea regulation under a law President Obama signed last year.

However, Spencer worries about the cost of getting each driver or conductor tested.

"Just the cost of doing that test is over $2,600," he said. "That's a pretty big piece of change, and that's just for doing the test."

And medical testing is not the only solution. There is also a new low-speed, autonomous breaking device, already widely available in high-end cars, that has the technology to brake to avoid a collision and alert drivers as they start to nod off.

Spencer of the OOIDA is skeptical and believes "technology will never stop crashes," but the NTSB says these high-tech solutions, some of which are just beginning to be available as optional extras on trucks in the United States, hold promise.

"We specifically have looked at some things in rail and trucking and have made recommendations to find innovative technologies that could make a difference in alerting a driver to their fatigued state," Rosekind said.

Dr. Michael Caldwell of the ABC News Medical Unit contributed to this report